This invention relates to entrained flow gasifiers and, more specifically, to a method of operating such gasifiers.
A two-stage entrained flow gasifier is essentially comprised of a combustion zone and a reduction zone. In the combustion zone, coal is burned to generate hot combustion products which supply the heat required for the gasification process which takes place in the reduction zone. The combustion zone is operated at near stoichiometric conditions to obtain the maximum heat and also to melt the ash so that it may be removed in the form of slag. As applied to combustion and gasification, stoichiometric refers to the theoretical amount of oxygen or air required to completely burn the material being combusted. Off-stoichiometric refers to any ratio greater or less than the theoretical ratio while substoichiometric refers to a lesser amount of oxygen or air theoretically required for complete combustion. The operation of the combustion zone is off-stoichiometric only to the extent required to reduce the temperature within the combustion zone to a point which the materials forming the combustion zone can tolerate.
The combustion products formed in the combustion zone are conveyed into a reduction zone where they are mixed with additional pulverized coal. The additional coal is devolatized and the remaining carbon residue, termed char, reacts with the combustion products in the reduction zone to form a combustible gas which is largely carbon monoxide. The gasification reaction is an endothermic reaction obtaining its heat from the combustion products formed in the combustion zone. The gasification process continues within the reduction zone until the temperature is reduced to a level at which the gasification reaction rate is too slow for practical purposes. Any remaining coal particles in the form of char are removed from the combustible gas leaving the reduction zone and recycled either to the combustion zone or the reduction zone.
If the proper gasification reactions are to occur, it is imperative that a reducing atmosphere be maintained in the reduction zone. In most prior art entrained flow gasifiers, the additional coal injected into the reduction zone is typically conveyed from a pulverized coal bin to the reduction zone in a stream of air. Unfortunately, when the air enters the reduction zone, it preferentially reacts with the carbon monoxide generated in the gasification process rather than reacting with the additional coal injected into the reduction zone. This results in a poor gasification process and a lower heating value for the combustible product gas produced in the reduction zone. The use of air to convey the coal being injected into the reduction zone also limits the maximum heating value obtainable with an entrained flow gasifier.
One solution to this problem is presented in U.S. Pat. No. 3,454,383. As disclosed therein, the coal being injected into the reduction zone is conveyed from the coal source to the reduction zone in a stream of hot product gas taken from a point downstream of the reductor zone. Because the product gas itself is now the medium in which the coal is conveyed to the reduction zone, there is no decrease in heating value of the product gas due to oxygen in the conveying medium reacting with the carbon monoxide in the product gas. However, a major problem associated with this scheme is that the hot product gas is a combustible which could conceivably explode if allowed to reach its ignition temperature during the process of conveying coal to the reduction zone.
Another scheme for solving the above-mentioned problem is presented in U.S. Pat. No. 4,017,269. As disclosed therein, an inert gas is used as the medium to carry coal from the coal source to the reduction zone. Because the conveying medium is inert gas, there is no reaction with the product gas generated within the reduction zone as there would be if air were used as the conveying medium. Further, the hazard of an explosion which would exist if production gas is used as the conveying medium is avoided by using inert gas as the conveying medium. Nevertheless, the heating value of the product gas generated within the reduction zone is lowered because the inert gas tends to dilute the product gas. That is, the inert gas tends to absorb heat which would otherwise be part of the heating value of the product gas formed in the reduction zone, as well as adding unnecessary inert volume to the product gas.